The invention relates to a pneumatic brake booster for actuating a brake master cylinder of a motor vehicle.
The invention relates more particularly to a pneumatic brake booster for actuating a brake master cylinder of a motor vehicle, of the type comprising a rigid casing in which there can move a transverse partition sealingly delimiting a front chamber subjected to a first, engine depression, pressure and a rear chamber subjected to a second pressure that varies between the engine depression and atmospheric pressure, of the type which comprises a moving piston secured to the moving partition, a front face of which can act on a rod for actuation of the master cylinder, of the type comprising a rod for control of the booster, moving in the piston selectively as a function of an axial input force exerted forwards against a return force exerted on the rod by a return spring (50), of the type comprising a plunger which is arranged in front of the control rod in the piston and which at its rear end comprises at least one annular rear seat for a three-way valve which can move progressively between a position in which, with the control rod at rest, the front chamber and the rear chamber are in communication, and a position in which, with the control rod actuated, the second pressure in the rear chamber increases, the valve placing the rear chamber (18) in communication with atmospheric pressure, and of the type comprising a first feeler forming the front end of the plunger and passing through the piston which, when the control rod is in the rest position, is arranged a first determined jump distance away from a reaction disc inserted between the rod for actuation of the master cylinder and the front face of the moving piston, and which is able, when the control rod is actuated at a rate lower than a determined rate, to be made by the plunger to cover the first jump distance then to penetrate the reaction disc in such a way as to transmit to the plunger and to the control rod the reaction force of the master cylinder.
Numerous examples of conventional boosters of this type are known.
In such a booster, the distance separating the first feeler from the reaction disc is known as the “jump distance” and corresponds to the theoretical distance that the first feeler has to cover before the driver of the vehicle feels the reaction force of the master cylinder.
In practice, when there is clearance between the first feeler and the reaction disc, this distance corresponds to the distance separating the first feeler from the reaction disc, but it may also, when the reaction disc is initially decompressed in contact with the reaction disc, correspond to the travel over which the first feeler is likely to compress the reaction disc until the latter is fully compressed.
In an extreme braking situation for which maximum braking force is exerted on the control rod, actuation of the control rod causes movement of the plunger comprising the first feeler, and this causes the three-way valve to open wide and the rear chamber to be placed at atmospheric pressure. This results in a forwards movement of the moving partition, and the end of the plunger forming the first feeler penetrates the reaction disc made of elastomer which is secured to the rear face of the moving piston, compressing it.
Thus, the force exerted on the rod for actuation of the master cylinder when the control rod is at the end of its travel is the result of the boost force which is brought about by the pressure difference across the moving wall and the force exerted by the plunger forming a feeler on the reaction disc. The driver of the vehicle also feels the braking reaction force, which is transmitted from the master cylinder to the plunger via the reaction disc.
Now, it has been found that a good many drivers, when confronted with an emergency braking situation, underestimate the risks involved and, having braked sharply, release their braking force even though maintaining a braking force is essential in order to avoid an accident.
What happens is that, in the case of an extreme braking situation accompanied by a swift movement of the control rod, the plunger may come into contact with the reaction disc and transmit to the driver a feeling of maximum braking before the pressure difference across the front and rear chambers reaches its maximum, and this may lead the driver to release his force, even though he ought to maintain it in order to benefit from maximum braking force.
In order to remedy this drawback, a booster has been proposed which comprises a first feeler which is slidably mounted with respect to the plunger to penetrate the reaction disc and which can then, when the control rod of the booster is actuated at a determined rate, be locked with respect to the moving piston so as to maintain maximum braking force on the rod for actuation of the master cylinder via the reaction disc even though the driver may have partially released his force.
This design has the disadvantage of entailing a significant force on the part of the driver, because in order for the feeler to penetrate the reaction disc at the end of its travel, the actuating force has to be exerted against the reaction force of the master cylinder.
This is particularly penalizing in the case of emergency braking, that is to say when a braking force is applied swiftly when the duration of actuation needs to be as short as possible.